Recently, solid electrolytes are used more often than the conventional electrolyte (aqueous) solutions. This is because firstly those solid electrolytes have good processability in application in electric and electronic components, and secondly there are trends for overall size and weight reduction of such components and further for power saving.
Proton conductive materials, both inorganic and organic, are known in the art. However, inorganic proton conductive compounds, such as uranyl phosphate hydrate, come with many difficulties when superposed as a conductive layer onto a substrate or an electrode. For example, sufficient contact cannot be achieved in the interface between the conductive layer and the substrate or the like.
On the other hand, the organic proton conductive compounds include organic polymers that belong to the so-called cation exchange resins, for example sulfonated vinyl polymers such as polystyrene sulfonic acid; perfluoroalkylcarboxylic acid polymers and perfluoroalkylsulfonic acid polymers represented by Nafion® (DuPont); and polymers occurring by introducing sulfonic or phosphoric groups in heat resistant polymers such as polybenzimidazole and polyether ether ketone (Polymer Preprints, Japan, Vol. 42, No. 7, p. 2490–2492 (1993), Polymer Preprints, Japan, Vol. 43, No. 3, p. 735–736 (1994), Polymer Preprints, Japan, Vol. 42, No. 3, p. 730 (1993)).
These organic polymers are generally in the form of film when used as electrolytes. Their solvent solubility and thermoplasticity enable them to form a conductive membrane jointly on an electrode. However, many of the organic polymers are still insufficient in proton conductivity. In addition, they have poor service durability, reduce proton conductivity at high temperatures (100° C. or above), are embrittled by sulfonation to cause low mechanical strength, and have high moisture dependence. Moreover, the adhesion thereof with an electrode is not satisfactorily good. Further, because of the water-containing structure of these polymers, the conductive membranes are excessively swollen during operation, resulting in lowered strength and deformation.
U.S. Pat. No. 5,403,675 discloses a solid polymer electrolyte comprising a sulfonated rigid-rod polyphenylene. This polymer mainly contains an aromatic compound composed of phenylene units and has been sulfonated by reaction with a sulfonating agent to introduce therein sulfonic groups. Although increasing the amount of the sulfonic groups introduced improves the proton conductivity, it also results in remarkably deteriorated hot water resistance and toughness.